Press the 5th fret on your A string, and then strike both it and the D string. If they're not in tune, you'll hear the notes oscillate in and out of phase with each other. The closer to proper tuning they're in, the further apart those oscillations will be, until you just hear the same note.

It's even easier if you tune using harmonics. If you know how to play harmonics, you can play the 5th fret harmonic on the A string, and the 7th fret harmonic on the D string, and it's even easier to hear the notes ring if they're not matched up.